1. Technical Field
The present invention relates to a resonant circuit, a method for producing the resonant circuit, and an electronic device. Particularly, the invention relates to a structure of a resonant circuit utilizing a MEMS resonator and a method for producing the resonant circuit, as well as to an electronic device including the resonant circuit.
2. Related Art
In the recent electronic device market, products using MEMS (micro electro-mechanical system) technology are becoming increasingly popular. The MEMS products are electro-mechanical devices (MEMS devices) having a micro structure (a MEMS structure) formed on a substrate by using a semiconductor-manufacturing technology. Specific examples of the devices using the MEMS technology include micro sensors such as acceleration sensors, angular velocity sensors, inertial sensors, and pressure sensors. Market demands for the sensors are rapidly growing these days. Thus, the MEMS technology has great potential in creating new devices. In the future, a variety of practical applications of the MEMS devices, alone or in combinations, are expected to be sequentially achieved by utilizing characteristics of micro structures such as micro mechanical relays (switches) and variable capacitance elements.
A new practical example of the MEMS technology is a resonant circuit including a MEMS resonator. The resonant circuit with the MEMS resonator is typically operated by an electrostatic driving and capacitive sensing system or a piezoelectric driving and piezoresistive sensing system. Particularly, the former system is highly consistent with a semiconductor process such as a CMOS process and thus can be regarded as an effective system for miniaturization and cost reduction. In the electrostatic driving and capacitive sensing system, there is formed a MEMS resonator having a movable electrode and a fixed electrode to sense electrostatic force-induced vibration of the movable electrode by a change in capacitance between the movable electrode and the fixed electrode. FIGS. 3A and 3B show common examples of transmission characteristics and phase characteristics of a resonant circuit. Examples of the resonant circuit shown in the drawings are disclosed in U.S. Pat. Nos. 6,249,073, 6,424,074, and JP-A-2004-58228.
In the resonant circuit described above, the resonance frequency is changed by structural size variation in the MEMS resonator. Accordingly, to secure precision of resonance frequency, it is necessary to reduce variations in structural sizes of the resonator in production of the resonant circuit. If the variations cannot be reduced, the resonance frequency needs to be corrected at a trimming step or the like performed after the production. This makes a production process of the resonant circuit complicate, leading to an increase in production cost.
Among the structural sizes of the MEMS resonator, particularly, a thickness of a movable portion of the movable electrode has directly great influence on the resonance frequency. In addition, the movable portion has an extremely small thickness of a few um or smaller, so that even a small thickness variation causes a significant change in the resonance frequency.